Exchange systems wherein at least one bundle of hollow fibers is utilized are used in various ways for the exchange of a substance between two fluids. The one fluid flows on the outer side of the hollow fibers in a first flow space whereas the other fluid is conducted in the second flow space defined by the inner spaces of the hollow fibers. In actual use, so-called hollow-fiber modules are mostly used with each module having a housing in which the hollow fibers extend and wherein the first flow space is formed between the inner wall of the housing and the outer surfaces of the hollow fibers. A substance exchange system in the sense of this invention can comprise one or several of such hollow-fiber modules. Several modules form a system when they are connected with each other so that they together serve a specific substance exchange purpose.
Hollow fiber substance exchange systems are utilized for various purposes especially in the medical area such as for dialysis equipment or hemofiltration equipment. A further area of application is for the humidification of air which can, for example, be used in connection with apparatus for artificial respiration. Such breathing-air humidifiers are disclosed in U.S. Pat. Nos. 4,146,597 and 4,318,398 as well as in European Patent Publication No. 0,009,543 which corresponds to U.S. patent application Ser. No. 923,905 filed on July 12, 1978 and U.S. patent application Ser. No. 046,943 filed on June 8, 1979. The invention is especially directed to such substance exchange systems for the humidification of gases such as breathing air.
The walls of hollow fibers preferably comprise semi-permeable membranes which make possible a selective transfer of substances from one fluid into the other. For the humidification of breathing air, water flows through the first flow space. The breathing air is conducted through the second flow space, that is, the breathing air is conducted through the interior of the hollow fibers. The water is usually heated. The hollow fibers are permeable for water molecules so that the breathing air flowing in their inner space is humidified. The degree of humidification is dependent upon the material characteristics of the hollow fibers and is especially dependent upon the magnitude of the pores of the fibers and upon the dimensions (wall thickness and diameter) of the fibers in the module. These influencing quantities are here grouped together and identified as the substance exchange characteristic values of the hollow fibers. In addition, the degree of humidification of the breathing air is also dependent upon the temperature of the water flowing through the first flow space because its vapor pressure increases with increasing temperature.
In the area of the humidification of breathing air, it is intended that the breathing air be warmed to a temperature which is favorable for the patient. The degree of heating is dependent upon the heat conductive capacity of the hollow fibers and upon the dimensions (wall thickness and diameter) thereof. These can be collectively referred to as the heat exchange characteristic values. In addition, the heating of the breathing air is dependent upon the temperature of the water.
If the temperature of the water flowing through the first flow space is changed, then the quantity of heat exchanged also changes as does the exchanged quantity of water in correspondence to the particular exchange characteristic values. As a consequence of the foregoing, it is not possible to vary the moisture content and the temperature of the breathing air independently of each other. This, however, would be desirable because from a medical point of view, different proportions with respect to moisture and temperature of the breathing air should be possible for various patients and for various application purposes. The moisture and the temperature of the breathing air should be changeable during the operation of the system. Similar problems exist in other areas wherein the substance exchange module can be utilized and wherein an undesired coupling of the exchange of different substances and/or energies exists because of the given exchange characteristic values.